As it is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing a laminated web of packaging material.
The packaging material has a multilayer structure substantially comprising:                a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and        a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.        
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a tube is formed continuously from the web-fed packaging material. More specifically, the web of packaging material is unwound off a reel and fed through an aseptic chamber on the packaging machine, where it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is subsequently evaporated by heating, and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity. The web of packaging material so sterilized is then maintained in a sterile-air environment, and is vertically fed through a number of forming assemblies which interact with the packaging material to fold it gradually from web form into a tube shape.
Afterwards, the tube is filled with the sterilized or sterile-processed food product and is sealed and cut at equally spaced cross sections into pillow-pack packages, which are subsequently folded mechanically to form parallelepiped packages.
Before reaching the forming assemblies, the web of packaging material is fed through an apparatus for applying a sealing strip of heat-seal plastic material, to which the packaging material is subsequently heat sealed to form the vertical tube.
More specifically, the sealing strip is normally heat sealed to the inner plastic layer of the web of packaging material at a pressing station to which the web of packaging material and the sealing strip, both heated beforehand, are fed along different paths, and where the sealing strip is pressed onto a first longitudinal edge of the web of packaging material. After the application, the sealing strip has a first portion heat sealed to the first longitudinal edge of the web, and a second portion projecting therefrom.
On interacting with the forming assemblies, the second longitudinal edge of the web is laid on the outside of the first longitudinal edge with respect to the axis of the tube being formed. More specifically, the sealing strip is located entirely inside the tube, and the face of the second longitudinal edge facing the axis of the tube is superimposed partly on the second portion of the sealing strip, and partly on the face of the first longitudinal edge located on the opposite side to the first portion of the sealing strip.
Apparatus of the above type are known in which the first and second longitudinal edge are heat sealed to form a longitudinal seal along the tube; more specifically, the heat-seal operation comprises a heating step to heat the second longitudinal edge without the sealing strip, and a pressure step to compress the sealing strip and the longitudinal edges.
The heating step melts the polyethylene layer of the second longitudinal edge, which transmits heat by conduction to the first longitudinal edge and the sealing strip, so as to melt the polyethylene layer of the first longitudinal edge and the heat-seal material of the sealing strip.
At the pressure step, the sealing strip and the longitudinal edges of the web of packaging material are pressed together, so that the heat-seal material of the sealing strip and the polyethylene layers of the longitudinal edges blend completely and form the molecular bonds defining the longitudinal seal of the tube.
The sealing strip performs the following functions:                to prevent the edges of the packaging material forming the longitudinal seal to absorb the packed product;        to improve the gas barrier properties of the longitudinal seal area;        to strengthen the longitudinal seal.        
In order to properly work, the sealing strip should be correctly positioned with respect to the packaging material.
A need is therefore felt within the industry to control continuously and accurately the position of application of the sealing strip onto the web of packaging material without disturbing the functioning of the packaging machine.
A need is also felt within the industry to adjust the above-mentioned position when necessary, without stopping the packaging machine, to ensure correct application of the sealing strip onto the web of packaging material.